The present invention relates to a novel resistor composition having an excellent temperature coefficient of resistance (TCR) and its method of preparation. More particularly, the present invention is directed to a resistor composition containing a conductive material and a glass frit or a conductive material, a glass frit and an insulating or semiconductive metal oxide wherein the weight ratio of said conductive material and said glass frit or said conductive material, said glass frit and said metal oxide is maintained constant and the resistance value of the resistor composition is determined by varying the total surface area of said conductive material and said glass frit or by varying the total surface area of said conductive material, said glass frit and said metal oxide without substantially changing the temperature coefficient of resistance of the resistor composition.
In the previous, well-known techniques, the preparation of a resistor composition containing a series of varied resistance values was obtained by controlling the weight ratio of the components of the resistor composition, that is, the weight ratio of the conductive material and the resistive material. However, in following the well-known techniques for the preparation of resistor compositions, the variation of the resistance value was always accompanied by a simultaneous deviation in the temperature coefficient of resistance. Therefore, in the prior art resistor compositions and method of manufacture, it was impossible to obtain certain definite resistance values without varying the temperature coefficient of resistance.
In addition, although an even surface film resistor with higher resistance value is obtainable, adoption of some special devices are inevitably required in the preparation processes of the resistor composition. With respect to resistors having a lower resistance value, although compositions having satisfactory printing ability are obtainable, the yielded resistors normally have uneven surfaces and also unstable resistance values.
The present invention is directed to a resistor composition comprising a conductive material and a glass frit, or a conductive material, a glass frit and an insulating or semiconductive metal oxide, wherein the weight ratio of the conductive material and the glass frit and, when present, the said metal oxide is constant and the resistance value of the composition is determined by varying the total surface area of the conductive material, the glass frit and, when present, the said metal oxide, without changing the temperature coefficient of resistance of the composition. In the process for manufacturing the resistor composition of the present invention, conductive materials, glass frit and insulating or semiconductive metal oxides having known specific areas are utilized and the resistance value of the resistor composition is determined by increasing or decreasing the total surface area of said conductive material, said glass frit and, when present, said metal oxide, while maintaining the weight ratio of said conductive material, glass frit and said metal oxide constant. Alternatively, the specific surface area of one or two components selected from the above two or three components can be either increased or decreased while maintaining the specific surface area of the residual component constant and while maintaining the weight ratio of said two components or said three components constant. Advantageously, a vehicle is provided for said two-component or three-component resistor composition of the present invention.
Thus, according to the present invention, the problems encountered in the prior art resistor compositions and processes have been overcome by the teachings of the present invention which are summarized as follows:
1. A resistor composition comprising a conductive material, a glass frit and a vehicle therefor wherein the weight ratio of said conductive material to said glass frit is constant and the resistance value of said composition is established by varying the total surface area of said conductive material and said glass frit without changing the TCR of said composition.
2. A resistor composition comprising a conductive material, a glass frit, an insulating or semiconductive metal oxide and a vehicle therefor, wherein the weight ratio of said conductive material, said glass frit, and said insulating or semiconductive metal oxide is constant and the resistance value of said composition is established by varying the total surface area of said conductive material, said glass frit and said insulating or semiconductive metal oxide without changing the TCR of said composition.
3. A process for manufacturing a resistor composition characterized by using a conductive material and a glass frit having known specific surface areas, respectively, and establishing the resistance value of said resistor composition by increasing or decreasing the total surface area of said conductive material and said glass frit while maintaining the weight ratio of said conductive material to said glass frit constant.
4. A process for manufacturing a resistor composition characterized by using a conductive material, a glass frit and an insulating or semiconductive metal oxide having known specific surface areas, respectively, and establishing the resistor value of said resistor composition by increasing or decreasing the total surface area of said conductive material, glass frit and insulating or semiconductive metal oxide while keeping the weight ratio of said conductive material, glass frit and insulating or semiconductive metal oxide constant.
5. A process for manufacturing a resistor composition characterized by using a conductive material and a glass frit having known specific areas, respectively, and establishing the resistance value of said resistor composition by increasing or decreasing the specific surface area of one component and maintaining the specific surface area of the other component constant while keeping the weight ratio of said conductive material to said glass frit constant.
6. A process for manufacturing a resistor composition characterized by using a conductive material, a glass frit and an insulating or semiconductive metal oxide having known specific surface areas, respectively, and establishing the resistance value of said resistor composition by increasing or decreasing the specific surface area of one or two components and maintaining the specific surface area of the residual component or components constant while keeping the weight ratio of said conductive material, glass frit and insulating or semiconductive metal oxide constant.
As mentioned above, one of the main features of the present invention is that a definite resistance value is easily obtained by controlling the total surface area while the temperature coefficient of resistance is maintained substantially constant. However, the theoretical reasons why this phenomena exists is uncertain. One possible assumption in this connection is an explanation based upon the contact area between the resistive material and the conductive material. But the effects cannot be fully understood from only the above assumption. In any event, the amount of reproducibility involved in the present invention suggests that this is an entirely novel and widely applicable technical contribution which has not yet been fully supported by theoretical bases.
The term "specific surface area" as referred to hereinabove shall be defined as the surface area of each 1 gram of finely divided particles, and accordingly, the "total surface area" can be defined by the following equation: EQU Total surface area = specific surface area .times. total wt. of particles
The conductive material or component which can be utilized in the present invention can be, for example, Au (gold), Ag (silver), Pt (platinum), Rh (rhodium), Ru (ruthenium), Os (osmium), Ir (iridium), V (vanadium), Sn (tin), W (tungsten), C (carbon), and alloys, mixtures, and oxides thereof. These conductive materials, after the composition has been fired, become highly conductive particles.
The glass frits which can be used in the resistor composition of the present invention are, generally speaking, conventional glass frits. Examples of such glass frits include the borosilicates and particularly the lead-borosilicates.
The insulating or semiconductive metal oxide which can be used in the resistor composition of the present invention should be capable of producing, after firing, finely divided particles with insulating or semiconductive properties. Exemplary of suitable insulating or semiconductive metal oxides include palladium oxide, copper oxide, aluminum oxide, zinc oxide, iron oxide, chromium oxide, cobalt oxide, tantalum oxide, nickel oxide, niobium oxide, silicon oxide and the like. The finely divided particles of the said conductive material, glass frit and metal oxide are those containing a diameter of about 100 A to 50 .mu..
The vehicle which can be used in combination with the conductive material, glass frit and the insulating or semiconductive metal oxide in forming the resistor composition of the present invention can be an organic binder, such as, for example, ethyl cellulose, alkyd resins, butyral resins, nitrocellulose, and the like. Any vehicles which are normally used in the resistor field of technology are applicable to the resistor composition and method of the present invention.
Examples of suitable solvents which can be included in the resistor composition of the present invention include organic solvents such as butyl carbitol, butyl carbitol acetate, terpineol, tetralin, and the like.
In the resistor composition of the present invention, the conductive material can be present in an amount of about 10 to 60 parts by weight and the resistive material, which includes the glass frit alone or the glass frit and the insulating or semiconductive metal oxide can be present in an amount of about 40 to 90 parts by weight.
The specific surface area of the conductive material, glass frit and insulating or semiconductive metal oxide can be varied from 0.02 to about 270 m.sup.2 /g. Within this range, the specific surface area of the conductive material can vary from about 0.02 to about 85; the specific surface area of the glass frit can vary from about 0.05 to 2.0, and the specific surface area of the insulating or semiconductive metal oxide can vary from about 0.5 to 265.